To allow for greater flexibility and ease of use, portable devices or battery operated consumer electronics devices, such as laptop computers, smartphones, and tablet computers, for example, allow the user to flip an external power source plug. In other words, the polarity of the dc voltage received at the connector of the computer can be reversed. A power manager circuit in the computer can automatically detect the particular polarity and then configure itself accordingly, so that the user need not pay attention or be concerned with the orientation of the plug. The plug may be attached to a battery charging cable that connects the portable device to an external dc power source, such as an ac wall power adapter. The power manager circuit in the computer often has a bridge rectifier circuit having a pair of input terminals that are connected to a dc input circuit and a pair of output terminals that will supply a voltage of known or fixed polarity to a load regardless of the polarity of the input voltage. One of two alternate current paths is automatically enabled, depending upon the polarity of the input voltage
When the bridge rectifier is implemented using diodes (a “diode bridge”), relatively large power losses are incurred due to the p-n junction or diode voltage drops that it requires. This is of concern when the dc input source has a lower voltage. For example, if the source is 5 Volts dc, and the diode bridge circuit presents a voltage drop of 0.7 Volts per diode, then 1.4 volts of the total 5 Volts that is available will be lost to the diode bridge, so that the voltage efficiency is a relatively poor (5−1.4)/5=72%. To mitigate this problem, the bridge rectifier may be implemented instead with low loss active devices such as insulated gate field effect transistors that are switched on and switched off to configure or create the correct current path, depending upon instructions received from a separate polarity sensing circuit. The latter senses the polarity of the dc input source, and in response provides the needed control signals to switch on and switch off the correct current path through the field effect transistors that make up the rectifier.